The present invention relates to the shaping and vulcanizing of curing bladders used in tire manufacture.
For several decades tire manufacturers have employed automatic vulcanizing presses, such as the McNeil "Bag-O-Matic" presses shown in various patents of L. E. Soderquist. These presses employ butyl rubber curing bladders which fit within the tire during the vulcanizing operation and which are readily removed from the finished tire at the end of each curing cycle. One curing bladder can be used to cure 200 to 300 tires or more.
During the last 30 years attempts have been made to improve the useful life of the curing bladders and to minimize the cost of shaping and vulcanizing such bladders. The tire industry has employed standard types of vulcanizing molds which facilitate economical production of the curing bladders and which employ split cores to help in removal of the cured bladder from the core at the end of the cycle.
The lower section of the split core, known as the "core plate", fits the main upper section of the core and fits within the lower mold half. The upper section fits within the upper mold half and extends into the lower mold half. When the mold is new the two core sections will register properly with the upper and lower mold halves and the curing bladder can be formed with the desired uniform wall thickness. However, normal wear and metal distortion resulting from extended use of the mold causes the two core sections, when registered together, to be too small for the space provided for them within the two mold halves, and the result is that the core assembly will register against either the top or bottom mold half and the bladder will have non-uniform wall thickness.
Another result of such wear is that the core plate will register against the bottom mold half and the main core section will register against the top mold half so as not to be properly located relative to the core plate. This promotes rubber stock intrusion into the gap left between the main core section and the core plate which will produce a thin fin of substantial width on the inside of the finished bladder. Even when the mold parts fit together perfectly, the forces of stock flow during molding can cause the main core section and the core plate to separate and allow such intrusion of the stock into the gap between the core sections.
The above problem cannot be solved merely by enlarging the core or reducing the clearances because the core does not have the strength required to support the tremendous forces applied by the hydraulic press. Clearances are essential to prevent fracture of the core, and molds are commonly designed to allow the registering surfaces in the mold halves to deform rather than to transmit excessive forces that could fracture the core. This design in turn leads to early wear or enlargement of the clearances and return to the problems described previously.
Despite these problems the use of a split core with a core plate (rather than a one-piece core) has been necessary to permit removal of the finished curing bladder from the core and to permit economical production. The core plate is necessary to hold the finished curing bladder in the bottom mold half while the main upper core section is pulled out of the bladder. The core plate is then lifted from the bottom mold half to permit removal of the bladder.
The bladder molds described above have been standard in the industry for many years and have been considered the most practical in spite of the above problems. The use of these molds has limited the quality of the curing bladders and their useful life and has made it difficult to produce thin-wall bladders with satisfactory service life (i.e., bladders with a wall thickness below 0.2 inch).
The flow of rubber is seriously disturbed as the result of rubber stock intrusion into the gap between the core plate and the main core section (see the photograph of FIG. 10). The rubber forced into this gap becomes partially cured and is then forced out of the gap back into the mold cavity so as to form a defect in the bladder wall. Such defects are a major cause of failure.